The object of this invention is to produce a 1,4-O-metallation product of the general structure (C) by reacting an .alpha.,.beta.-unsaturated carbonyl compound (i.e., an ester, ketone or aidehyde) of the general structure (B) with a hydride of silicon, of the general structure (A) in the presence of a heterogeneous noncomplexed rhodium-containing catalyst. The process involves the following single step reaction in which the carbon and oxygen positions are numbered. ##STR1## The reaction can be carried out in a solvent or neat, can be run with or without a polymerization inhibitor, can be run at mild reaction conditions (temperatures ranging from about 10.degree. C. to about 75.degree. C. and pressures from about atmospheric to about 20 psig), and yields a high purity product from which the catalyst can easily be separated for recovery or reuse.
The 1,4-O-metallation products, namely enol ethers of the form ##STR2## and ketene acetals of the form ##STR3## and of the form ##STR4## wherein M is silicon, are a useful class of compounds. They are reagents of choice in a wide range of reactions, including those which do not involve carbon-carbon bond formation, heterolytic reactions with carbon-carbon bond formation and pericyclic reactions. Some of these compounds are particularly useful as initiators in the manufacture of polymers for lacquers, oils or rubbers. See U.S. Pat. No. 4,417,034 issued to O. W. Webster on Nov. 22, 1983, and U.S. Pat. No. 4,508,880 issued to O. W. Webster on Apr. 2, 1985.
Compounds of the general structure ##STR5## wherein M is silicon, and either R.sup.2 or R.sup.3 or both R.sup.2 and R.sup.3 are perfluoroalkyl and compounds of the general structure ##STR6## wherein M is silicon, R.sup.3 is perfluoroalkyl and no other R group is perfluoroalkyl are new compounds which can be synthesized by the process of this invention.
This invention is also useful for the production of 1,4-O-metallation polyfunctional initiators to be used in making higher molecular weight polymers than would be possible with 1,4-O-metallation products of the form shown above. These polyfunctional initiators are of the form ##STR7## wherein M is silicon.
Representative of the 1,4-O-metallation products and processes for making them is the 1,4-O-silylated product, wherein M is silicon. In the past this type product has been synthesized by one of three methods--the silylation route, the enolate route, and the hydrosilylation route.
The silylation route is described by Rhone-Poulenc S.A. in its British Patent No. 1,044,448. They describe the silylation of an organic compound in the presence of a nickel catalyst. The organic compound must contain an enolizable carbonyl group but be free from other functional groups which are reactive under the reaction conditions. The reaction is accompanied by the evolution of hydrogen, and may be represented as follows: ##STR8##
The reaction taught in the British Patent is limited to enolizable aldehydes and ketones. It also involves the evolution of hydrogen.
The enolate route is a three-step reaction which involves the following:
1. Generation of lithium diisopropylamide (LDA),
2. Reaction of the LDA in an organic solvent with the appropriate carbonyl compound to form the enolate, and
3. Reaction of the enolate with a halosilane to form the silylated product.
To isolate the silylated product prepared by the enolate route, filtration to remove salts, evaporation of solvent and distillation of the product are necessary. The process uses large quantities of flammable solvent and a pyrophoric material n butyl lithium to generate LDA. Also, the process suffers from low volumetric efficiencies (less than about 0.5 pound of product per gallon of raw materials) and it must be run at low temperatures (typically less than 0.degree. C.).
The hydrosilylation route involves the reaction of silicon hydrides with .alpha.,.beta.-unsaturated carbonyl compounds. Reaction products of the hydrosilylation can be 1,2 .alpha.-silylated products, 1,2 .beta.-silylated products, 3,4-O-silylated products, or 1,4-O-silylated products.
MacKenzie, et al., describe the hydrosilylation route in their U.S. Pat. No. 2,721,873. That patent specifies the use of a silicon compound having at least one hydrogen attached to the silicon and an unsaturated organic compound containing the unsaturation in a non-benzenoid group to form an organo silicon compound. The silicon compounds utilized may be inorganic or organic. The unsaturated organic compounds include unsaturated hydrocarbons, aliphatic, carbocyclic, alicyclic and heterocyclic compounds including unsaturated alcohols, aldehydes, ketones, quinones, acids, acid anhydrides, esters, nitriles, or nitro compounds. The presence of added catalyst is nonessential, but MacKenzie, et al. say that they may be employed under certain conditions to facilitate reaction or increase yields. MacKenzie, et al. suggest that these catalysts may be selected from compounds and salts in the elements of groups IIIA, IVA, IB and IIB of the periodic system. Group VIII and some of their compounds are suggested as possibilities. Other types of catalysts such as peroxides are indicated and are cited as influencing the direction of addition which takes place.
Prior to this invention, 1,4 addition of silicon hydrides to .alpha.,.beta.-unsaturated carbonyl compounds has been shown to occur in the presence of a homogeneous or soluble catalyst such as tris(triphenylphosphine)rhodium(I) chloride. Since the catalyst is homogeneous, the product must be separated from the catalyst by distillation in order to recover the precious metal.
The process of this invention comprises reacting a hydride of silicon with .alpha.,.beta.-unsaturated esters, ketones or aldehydes in the presence of a heterogeneous noncomplexed rhodium-containing catalyst, that is, a solid catalyst in a gas or liquid system, to make a predominantly 1,4-O-metallation product. After the reaction is complete, the rhodium-containing catalyst can be easily removed by filtration to recover the precious metal. The crude product of this process, after filtration, can be distilled to isolate the 1,4-O-metallation product. In some cases distillation is not required for the product to be useful.
Polymerization inhibitors such as hydroquinone, tetramethyldiphenoquinone, phenothiazine and p-methoxyphenol can be used to retard polymerization of the unsaturated hydrocarbon. These are particularly advantagteous when a gaseous hydride such as trimethylsilane is added to a slurry of the .alpha.,.beta.-unsaturated carbonyl compound and the heterogeneous rhodium catalyst.
While the process of this invention can be run neat, the reaction can be run in a solvent such as tetrahydrofuran (THF), ethyl acetate, or ethylene glycol dimethyl ether (glyme). For certain 1,4-O-metallation reactions, fewer impurities in the crude product result when a solvent is employed. Also, the choice of solvent can affect purity.